Phenol, one of the main components of phenolic resins, is a relatively expensive petrochemical. Because phenol is a petrochemical, its market price fluctuates with crude oil prices. Since crude oil prices have been and are expected to remain high, there continues to be an impetus to find non-petrochemical based replacements for raw materials, such as phenol, derived from crude oil.
A major use of phenol is in the manufacture of phenol-formaldehyde resins. Such resins are used in large quantities as adhesives, coatings, fillers and the like in the manufacture of plywood, particle board and other composite wood products. Due to the high cost of the raw materials derived from petrochemical sources, such adhesives, coatings, fillers and the like are relatively expensive, relative to the value of the finished product. For these reasons, there is a strong economic incentive to replace as much of the phenol as possible with a less costly modifier that does not detract from the resin performance.
Because of the similarity of its chemical structure to phenolic resins, numerous proposals have been made to modify kraft lignin, among other lignins, by reaction with chemicals which promote cross linking of the lignin molecules. Kraft lignin is a by-product of a particular alkali wood pulping process, the Kraft process. Such cross linked lignins may be used as modifiers in phenolic resins. One of the drawbacks of the prior art, which has limited the use of such modified lignins, is that large amounts of phenol still must be used to produce a resin with suitable properties. Large amounts of phenol are required because the reactivity of the lignin molecules is relatively low. It also has proved difficult to effect sufficient cross linking between the lignin and the phenol-formaldehyde resin to achieve good resin properties, and in particular properties of composite wood products certified for exterior use.
U.S. Pat. No. 3,017,303 describes the use of purified alkali lignin as a modifier for phenolic plywood resins. U.S. Pat. No. 3,658,638 discloses lignosulfonate as a phenol replacement in an adhesive resin made by co-condensing lignosulfonate, phenol and formaldehyde. U.S. Pat. No. 3,864,291 describes a plywood adhesive made by reacting black liquor of the kraft or soda alkaline pulping processes with formaldehyde and then blending this adduct with a phenol-formaldehyde resin. U.S. Pat. No. 4,113,675 similarly describes an adhesive prepared by reacting lignin from kraft or soda black liquor with formaldehyde under alkaline conditions followed by cross linking with a phenol formaldehyde resin. U.S. Pat. No. 4,303,562 describes an alkali lignin-based adhesive prepared by adding a phenol-water-lignin solution to a partially condensed phenol-formaldelhyde resin and then reacting the mixture under alkaline conditions. Unfortunately, with all of the above methods the low level of lignin reactivity severely limits the relative amount of lignin that can be added to the resin (and correspondingly the amount of phenol that can be replaced).
U.S. Pat. No. 4,105,606 purportedly overcomes this problem by using a high molecular weight alkali lignin isolated for example by ultrafiltration. According to the '606 patent, the initial lignin should be treated to recover a lignin fraction in which at least 40 wt. percent of the lignin has a molecular weight in excess of that of Glucagon as determined by gel chromatography. Stated in an alternative fashion, the patent teaches the removal of the low molecular weight lignin fraction and the use of the remaining high molecular weight lignin fraction (at least 45 percent of the lignin should have molecular weights in excess of 5,000) as an improved lignin modifier for phenol. There is no discussion as to any utility of the low molecular weight fraction. In fact, with the disclosure of "at least 35 percent, properly over 40 percent or 45 percent, and preferably over 50 percent by weight of the alkali lignins shall have molecular weights in excess of 5,000 as determined by gel chromatography" (or alternatively, "at least 40 percent, properly over 45 percent or 50 percent and preferably over 55 percent by weight of the alkali lignins have molecular weights in excess of that of Glucagon") this patent indicates that the lignin molecular weights should be as high as possible.
The use of ultrafiltration to remove or recover constituents from black liquor also is known in the art. U.S. Pat. No. 3,758,405 describes a process using ultrafiltration to remove color from black liquor. The liquor is ultrafiltered through a membrane with a pore size of from about 0.01 to 0.05 micron and the color components are recovered in the concentrate. U.S. Pat. No. 4,155,845 describes a process wherein the waste wash liquor of chlorine bleaching and sodium hydroxide bleaching operations is ultrafiltered after the addition of a flocculating agent. U.S. Pat. No. 4,670,098 describes a pulping process in which the pulping liquid is subjected to ultrafiltration during the pulping operation to remove constituents having a molecular weight above 3,500, preferably above 2,000 and most preferably above 1,500. The pulping liquid, from which the higher molecular weight constituents have been removed, is returned to the pulping process. The recovered concentrate, which contains the higher molecular weight constituents, is said to contain mostly lignin materials and is said to have various uses. The solids content of this concentrate, obtainable by evaporation or spray drying, is said to be useful, inter alia, in the preparation of adhesives.
Low molecular weight lignin-containing by-products obtained by pulping of wood, typically hardwood, in an aqueous ethanolic liquor have been substituted for phenol in phenol-formaldehyde resins used for bonding maple blocks. An aqueous mixture of the lignin was reacted with the quantity of phenol required to form the resin. Then, the formaldehyde was added. The resulting resin was used as the bonding agent for the blocks.